1. Field of the Invention
This invention relates to an image forming device, such as a copier, a printer, a facsimile or a multi-function peripheral, which forms an image using a tandem type image formation unit, a photoconductor drum drive controlling method which is adapted to control driving of a photoconductor drum motor for the image forming device, and a computer-readable recording medium which is adapted to incorporate the photoconductor drum drive controlling method.
2. Description of the Related Art
Conventionally, among tandem type image forming devices, an indirect-transfer tandem type image forming device and a direct-transfer tandem type image forming device are known.
In the indirect-transfer tandem type image forming device, toner images of yellow, cyan, magenta, and black are formed on respective photoconductor drums, and these images are primarily transferred to an intermediate transfer belt (which is an intermediate transfer body), so that a full color image is formed by superimposing the images of the four colors on the intermediate transfer belt. The full color image formed on the intermediate transfer belt is secondarily transferred to a printing sheet, thereby forming the full color image on the printing sheet.
On the other hand, in the direct-transfer tandem type image forming device, each of toner images of four colors is respectively formed on one of the photoconductor drums and these images are superimposed on a printing sheet which is attracted and transported by a transfer transport belt, so that a full color image is formed on the printing sheet.
In order to ensure a long life of the photoconductor drums in these image forming devices, the color photoconductor drums are separated from the intermediate transfer belt or the transfer transport belt at the time of monochrome image formation. At the time of color image formation, the color photoconductor drums are contacted to the intermediate transfer belt or the transfer transport belt. Hence, the number of the photoconductor drums that are made in contact with the intermediate transfer belt or the transfer transport belt differs between at the time of monochrome image formation and at the time of color image formation. For this reason, the load on the intermediate transfer belt motor which drives the intermediate transfer belt (or the load on the transfer transport belt motor which drives the transfer transport belt) also differs between at the time of monochrome image formation and at the time of color image formation.
In order to ensure a high throughput when a color image and a monochrome image are output in a mixed manner, the imaging system motors in these image forming devices are continuously operated without stopping them during the mixed outputting of the color and monochrome images. In this case, the fluctuations of the load on the intermediate transfer belt motor or the transfer transport belt motor become large when the photoconductor drums are separated from the intermediate transfer belt (or the transfer transport belt) or when they are contacted to the intermediate transfer belt (or the transfer transport belt), and a certain time must be taken until the speed of the intermediate transfer belt motor (or the transfer transport belt motor) is stabilized. There is a possibility that the speed of the intermediate transfer belt motor (or the transfer transport belt motor) at this time is not stabilized and the intermediate transfer belt motor (or the transfer transport belt motor) falls out of control.
To avoid the problem, in the tandem type color image forming device according to the related art, the imaging system motor is temporarily stopped before the photoconductor drums are separated from or contacted to the intermediate transfer belt (or the transfer transport belt), and then the imaging system motor is restarted in order to prevent the intermediate transfer belt motor (or the transfer transport belt motor) from falling out of control due to the load fluctuation.
Moreover, Japanese Laid-Open Patent Publication No. 2006-139063 discloses an image forming device provided with a rotation fluctuation preventing unit. The rotation fluctuation preventing unit is arranged to prevent the fluctuation of rotation of the belt-like member due to movement of the color photoconductor drums at a start of monochrome image formation in which the black photoconductor drum is contacted to the belt-like member and the color photoconductor drums are moved away from the belt-like member.
However, in the above-described tandem type color image forming device according to the related art, the load torque to the intermediate transfer belt motor (or the load torque to the transfer transport belt motor) differs between at a time of full-color image formation and at a time of monochrome image formation. Hence, it is difficult for the tandem type color image forming device according to the related art to stabilize the rotation of the motor which drives the intermediate transfer belt (or the transfer transport belt), for example, in a transition from full-color image formation to monochrome image formation.
In the image forming device of Japanese Laid-Open Patent Publication No. 2006-139063, the load on the belt-like member is controlled using the rotation fluctuation preventing unit. However, it is difficult to prevent the fluctuation of the load arising when the inertial load is connected, and the resulting fluctuation may cause deterioration of a reproduced image.